The present disclosure relates to a thin film transistor, manufacturing method of the same and electronic equipment and, more particularly, to a thin film transistor made of a crystalline oxide semiconductor, manufacturing method of the same and electronic equipment using this thin film transistor.
Recent years have seen rapidly increasing display size and frame rate. Further, the competition to develop three-dimensional (3D) display has gotten underway lately. Amid such a backdrop, upgrading the performance of thin film transistors (TFTs) used as pixel switching elements of a display is becoming a necessity.
In particular, an a-Si:H TFT using hydrogenated amorphous silicon (a-Si:H) as its channel layer, i.e., a TFT for use as the above TFT, is approaching its limit in terms of performance. Therefore, brisk efforts are afoot to develop the next generation of TFT materials (refer to Hideo Hosono, Solid State Physics 9, Vol. 44, No. 523, p. 621 (2009)). Of these, transparent amorphous oxide semiconductor (TAOS) materials and indium (In)-based TAOS in particular is drawing attention as a promising material. TAOS is known to be inexpensive and suitable for use over a large area because it can be formed at low temperatures and eliminates the need for steps such as laser annealing following the formation. As a matter of fact, the use of TAOS TFTs for 37-inch displays (LCDs) and 12.1-inch organic EL displays has been reported although this is still in a development stage (refer to J. K. Jeong et al., Soc. Inf. Display Digest 39, 1 (2008) and M.-C. Hung et al., TAOS 2010).
In order to commercialize this TAOS TFT, however, it is necessary to improve its reliability. That is, TAOS has a problem in that oxygen (O), one of the constituent atoms, splits off very easily. It is known that the characteristic changes such as shifting of the threshold voltage (Vth) occur if oxygen deficiency takes place as a result of the oxygen splitting off from the TAOS. Further, a high mobility of 30 cm2/Vs or more, higher than a mobility of up to 10 cm2/Vs obtained by TAOS, has begun to be sought after because of increasingly high definition and frame rate.
In order to solve these problems, approaches including the annealing of the In-based TAOS film after its formation and the formation of a protective film over the In-based TAOS film are used. However, it is difficult to completely suppress the TFT characteristic changes with these approaches. Basically, eliminating unstable bonds in the In-based TAOS film to the extent possible is considered necessary. Further, materials other than InGaZnO are searched for to improve the mobility.
The use of crystalline materials such as InZnO and InGaZnO4 for the TFT channel layer has been recently reported (refer to Japanese Patent Laid-Open Nos. 2008-311342 and 2011-142310). TFTs using such crystalline materials are expected to provide reduced characteristic variations as compared to TAOS TFT. The reason for this is that these TFTs are believed to be affected to a lesser degree by grain boundary scattering because the carrier conduction in In2O3-based materials is determined by 5 s orbitals (refer to Transparent Conductive Film Technology Compiled by the 166th Commission of Transparent Oxide Photoelectron Materials, the Japan Society for the Promotion of Science). However, the variations in TFT characteristics have yet to be completely suppressed to date.